1. Field of the Invention
This invention relates to an elongated printing head having a plurality of printing elements, a printing method and apparatus using this printing head, and an apparatus and method for correcting the printing head.
2. Description of the Related Art
A printing apparatus such as a printer, copying machine or facsimile machine prints an image comprising a dot pattern on a printing medium such as paper, a thin plastic sheet or cloth based upon image information. Among these printing apparatus, those which are the focus of attention because of their low cost make use of printing heads that rely upon the ink jetting method, the thermal printing method or LED method, etc., in which a plurality of printing elements corresponding to dots are arrayed on a substrate.
In a printing head in which printing elements such as heating resistors or nozzles are arrayed to correspond to a certain printing width, the printing elements can be formed through a process similar to that used to manufacture semiconductors. Accordingly, a transition is now being made from a configuration in which the printing head and driving integrated circuitry are arranged separately of each other to a configuration in which the driving integrated circuitry is formed on the board of the head on which the printing elements are arrayed. As a result, complications in driving the printing head be avoided and the printing apparatus can be reduced in size and cost.
Among these types of printing methods, the ink-jet printing method is particularly advantageous. According to this method, thermal energy is made to act upon ink and the ink is jetted by utilizing the pressure produced by thermal expansion. This method is advantageous in that the response to a printing signal is good and it is easy to group the discharge ports together at a high density. There are greater expectations for this method in comparison with the other methods.
However, when the printing head is manufactured through a process used to manufacture semiconductors, as mentioned-above, numerous printing elements to be made to correspond to the printing width are arrayed over the entire area of a board, and therefore it is very difficult to manufacture all of the printing elements without any defects. As a consequence, the manufacturing yield of the printing head is poor and this is accompanied by higher manufacturing cost. It is very difficult to achieve such a printing head in practice.
Accordingly, methods of manufacturing an elongated printing head have been disclosed in the specifications of Japanese Patent Application Laid-Open (KOKAI) Nos. 55-132253, 2-2009, 4-229278, 4-232749 and 5-24192 and in the specification of U.S. Pat. No. 5,016,023. According to these methods, a large number high-yield printing head boards, each having an array of a comparatively small number of printing elements, e.g., 32, 48, 64 or 128 printing elements, are placed upon a single heater board in conformity with the density of the array of printing elements, thereby providing an elongated printing head whose length corresponds to the necessary printing width.
It has recently become possible on the basis of this technique to simply manufacture a full-line printing head by arraying a comparatively small number (e.g., 64 or 128) of printing elements on a substrate and bonding these substrate (referred to as "heater board" or "element substrate") on which printing elements are arrayed, in a row on a base plate which serves as a base, in precise fashion in a length corresponding to the necessary printing width.
Though it has become easy to manufacture a full-line printing head, certain performance-related problems remain with regard to a printing head manufactured by the foregoing manufacturing method. For example, a decline in printing quality, such as irregular distribution, cannot be avoided. The cause is a variance in performance from one heater board to another heater board in the row of such heater boards, a variance in the characteristics of neighboring printing elements between heater boards and heat retained in each driving block of the printing elements at the time of printing.
In the case of an ink-jet printing head, not only a variance in the neighboring printing elements between the arrayed heater boards but also a decline in ink fluidity owing to the gaps between heater boards results in lower yield in the final stage of the head production process. For this reason, the state of the art is such that these printing heads are not readily available on the market in large quantities regardless of the fact these printing heads exhibit highly satisfactory capabilities.
FIG. 12 is a block diagram illustrating an example of the circuit construction of a heater board according to the prior art.
As shown in FIG. 12, numeral 900 denotes an element substrate (heater board) having heating elements (heating resistors) 901; power transistors 902 for controlling flow of current to the heating elements 901; a latch circuit 903 for latching printing data in sync with a latch clock (on pad 907); a shift register 904, the inputs to which are serial data (on pad 906) and a serial clock (on pad 905) synchronized to the serial data, for latching one line of data; a resistance sensor 914 manufactured by the same forming process as the forming process of the heating elements 901, for monitoring the resistance values of the heating elements 901; a temperature sensor 915 used to monitor the temperature of the heater board 900; and input/output terminals 905.about.913. Specifically, numeral 908 denotes a driving pulse input (heating pulse) terminal for externally controlling the ON time of the power transistors 902, namely the time during which a current is flowed through the heating elements (resistors) 901 to drive them. Numeral 909 denotes a driving power-supply (5 V) terminal for powering logic circuitry. Numeral 910 designates a ground terminal; 911 an input terminal for powering the heating elements 901; 912 terminals for driving and monitoring the resistance sensor 914; and 913 terminals for driving and monitoring the temperature sensor 915.
In the arrangement described above, serially entered printing data is stored in the shift register 904 and latched in the latch circuit 903 by a latch signal. In response to a heating pulse which enters from the terminal 908 under these conditions, the transistors 902 are turned ON in accordance with the printing data to flow a current through the corresponding heating elements 901, thereby heating ink in the respective ink passageways so that the ink is discharged from the ends of the nozzles in the form of droplets.
Consider the energy needed to form bubbles in the ink at the heating elements 901. If the thermal radiation conditions are constant, the energy will be expressed by the product of the necessary energy introduced per unit area of the heating element 901 and the surface area of the heating element 901. This means that the voltage across the heating element 901, the current flowing through it and the duration (pulse width) of current flow should be set to values according to which the necessary energy will be obtained. The voltage impressed upon the heating element 901 can be held substantially constant by supplying voltage from the power supply of the printing apparatus per se. As for the current flowed through the heating elements 901, the resistance values of the heating elements 901 differ depending upon the lot or board owing to a variance in the film thickness of the heating elements 901 brought about in the process for manufacturing the heater board 900. Accordingly, in a case where the applied pulse width is constant and the resistance value of a heating element 901 is greater than what the design calls for, the value of the current flowing through this heating element 901 declines and the amount of energy introduced to the heating element 901 becomes inadequate. As a result, the ink cannot be made to form bubbles properly. Conversely, if the resistance of a heating element 901 is too small, the current value will become greater than the design value even if the same voltage is applied. In this case, excessive energy is produced by the heating element 901 and there is the danger that the heating element 901 will burn out or have its service life shortened. A method of dealing with this is to constantly monitor the resistance values of the heating elements 901 by the resistance sensor 914 or the temperature of the heater board 900 by the temperature sensor 915, change the power-supply voltage or heating pulse width based upon the monitored values and arrange it so that a substantially constant energy is applied to the heating elements 901.
Next, consider the amount of ink discharged in the jetted droplets. The amount of discharged ink is related mainly to the volume of the ink bubbles. Since the volume of an ink bubble varies depending upon the temperature of the heating element 901 and the temperature of the surroundings, a pulse (a preheating pulse) whose energy is not high enough to jet ink is applied before the heating pulse that causes the jetting of the ink, then the temperature of the heating element 901 and of its surroundings is adjusted by changing the pulse width and output timing of the preheating pulse to thereby discharge ink droplets in a constant amount. This makes it possible to maintain printing quality.
Correction of variance in the resistance values of the heating elements 901 and control of the substrate temperature are carried out by feeding back signals from the respective sensors 914, 915 and outputting a heating signal whose heating pulse width, preheating pulse width and preheating/heating pulse timings have been altered based upon the feedback. However, in addition to the foregoing problems, there is a structural variance in the area of the orifice openings and a variance in the film thickness of a protective film provided on the heating elements 901. As a result, there is a variance in the amount of ink discharge produced by each nozzle. This leads to irregular density and streaks at the time of printing and makes it necessary to control the amount of ink discharge on a per-nozzle basis or in units of several nozzles. Furthermore, in a case where a plurality of the heater boards of FIG. 12 are placed in a row to construct an ink-jet head having a multiplicity of nozzles, the resistance values of the heating elements 901 differ from one heater board to the next. Consequently, the heating pulses for discharging ink must be changed for each heater board to bring the applied energies into conformity. In other words, in the case of a printing head constituted by a plurality of heater boards, irregular density is caused not only by the variance in orifice area but also by a conspicuous difference in density from one board to another. This means that correcting the amount of discharge on a per-nozzle basis within a heater board becomes more important in a printing head having a plurality of heater boards than in a printing head having a single heater board.